1. Field of the Invention
The present invention relates to a surface acoustic wave (hereinafter referred to as SAW) device that includes a joined substrate having a supporting substrate joined to a piezoelectric element substrate, and a method of manufacturing the SAW device.
2. Description of the Related Art
Surface acoustic wave (SAW) devices are being widely used today as bandpass filters for portable telephone devices or the like. Filters and resonators that utilize SAW devices are characteristically small in size and inexpensive. Therefore, SAW devices have become essential for small-sized communication devices such as cellular phones.
As cellular phones have become more and more sophisticated in recent years, there is an increasing demand for more sophisticated filters utilizing SAW devices. Since a SAW device normally exhibits variations in frequency with changes in temperature, the stability with temperature is expected to improve.
Conventionally, substrate materials for SAW devices include piezoelectric element substrates (hereinafter referred to as piezoelectric substrates). Particularly, lithium tantalate (hereinafter referred to as LT) and lithium niobate (hereinafter referred to as LN) are piezoelectric materials with large electric mechanical coupling coefficients, and are being widely used accordingly.
The piezoelectric materials such as LT and LN with large electric mechanical coupling coefficients, however, have a drawback in that the substrate characteristics are unstable with changes in temperature. On the other hand, piezoelectric materials such as crystal with excellent stability with temperature have a drawback in having small electric mechanical coupling coefficients. In general, the piezoelectric materials with large electric mechanical coupling coefficients have poor stability with temperature, while the piezoelectric materials such as crystal with excellent stability with temperature have small electric mechanical coupling coefficients. In view of this, a SAW device that includes a substrate made of LT (hereinafter referred to as “LT substrate”) can achieve wide-band filter characteristics, but is poorer in temperature stability than a crystal substrate or the like.
So as to compensate for the two contrary drawbacks and to realize a piezoelectric substrate that has a large electric mechanical coefficient and exhibits excellent stability with temperature, various techniques have been suggested. Yamanouchi, et al., IEEE Trans. on Sonics and Ultrasonics., vol. SU-31, pp. 51–57, 1984 (hereinafter referred to as Non-Patent Document 1), for example, discloses a technique of improving the stability with temperature by employing a substrate having a silicon oxide (SiO2) film formed on the surface of a LN substrate or an LT substrate. The silicon oxide film has the opposite temperature coefficient to that of the LN or LT substrate. Japanese Patent Publication No. 2516817 (hereinafter referred to as Patent Document 1) also discloses a technique of improving the stability with temperature by forming a polarity inversion layer with a thickness equivalent to the wavelength of SAW or smaller on the surface of an LT substrate. In this technique, a field short-circuiting effect is utilized to improve the stability with temperature. Japanese Unexamined Patent Publication No. 11-55070 (hereinafter referred to as Patent Document 2) and Ohnishi, et al., Proc. of IEEE Ultrasonics Symposium, pp. 335–338, 1998 (hereinafter referred to as Non-Patent Document 2), also disclose techniques of improving the stability with temperature by directly joining a thin piezoelectric substrate to a thick low expansion material substrate. In these techniques, expansion and contraction of the piezoelectric material with temperature changes are restricted to improve the stability with temperature. Yamanouchi, et al., Proc. of IEEE Ultrasonics Symposium, pp. 239–242, 1999 (hereinafter referred to as Non-Patent Document 3) further discloses a technique of improving the stability with temperature by joining a thin piezoelectric substrate to a thick low expansion material substrate with an adhesive agent or the like. In this technique, expansion and contraction of the piezoelectric substrate are also restricted to improve the stability with temperature. Japanese Unexamined Patent Publication No. 9-208399 (hereinafter referred to as Patent Document 3) discloses a technique of improving the SAW characteristics by joining two substrates of different kinds to each other by virtue of solid-phase reaction.
FIGS. 1A and 1B illustrate a conventional SAW device chip 100 that is formed with one of the joined substrates described above. As shown in FIG. 1A, four resonators (four one-port resonators) 10 are formed on one chip, and each two neighboring resonators 10 (A and B, C and D in FIG. 1A) form a filter. The SAW device chip 1 is of a duplexer type. In the conventional SAW device chip 100, comb-like electrodes (interdigital transducers: IDTs) 11 and reflection electrodes 12 that constitute the resonators 10 are formed on the upper surface of a piezoelectric substrate 20. In each resonator 10, one IDT 11 is sandwiched between two reflection electrodes 12. A supporting substrate is joined to the bottom surface of the piezoelectric substrate. Each two resonators 10 forming one filter are normally arranged not to overlap each other in the SAW propagating direction. So as to reduce the chip size, however, any two neighboring resonators 10 of different filters may be arranged to overlap each other in the SAW propagating direction.
In a case where a joined substrate is employed in a conventional SAW device, however, the joining interface between the piezoelectric substrate and the supporting substrate may be separated in two when the individual SAW device is cut out of a multiple substrate.
Japanese Unexamined Patent Publication No. 2001-60846 (hereinafter referred to as Patent Document 4) discloses a technique of solving the above problem by dicing the piezoelectric substrate and the supporting substrate with two different dicing saws. Referring now to FIGS. 2A through 2C, this conventional technique will be described in detail. In this technique, a joined substrate (a base substrate) is first formed with the piezoelectric substrate 20 having the IDTs 11 and the other components formed thereon and the supporting substrate 30, as shown in FIG. 2A.
When the base substrate is to be divided into individual SAW device chips 100, grooves 81 are formed by cutting (removing) the piezoelectric substrate 20 along the boundary lines of the individual SAW device chips 100, as shown in FIG. 2B, using a dicing blade. The supporting substrate 30 is then cut along the bottoms of the grooves and the boundary lines of the individual SAW device chips 100, as shown in FIG. 2C. The dicing blade used in this procedure is thinner than the dicing blade used in the procedure of FIG. 2B, or is thinner than the grooves 81. In this manner, the individual SAW device chips 100 can be obtained.
In a SAW device chip that has a supporting substrate with a smaller thermal expansion coefficient than that of the piezoelectric substrate so as to improve the stability with temperature, however, the piezoelectric substrate is made so thin as to restrict thermal expansion of the piezoelectric substrate by virtue of stress caused between the substrates. Because of this, most bulk waves that are generated from the IDTs formed on the piezoelectric substrate and are propagated in the depth direction are reflected by the joining interface. The degree of reflection is even greater in a case of a joined substrate formed by directly joining two substrates, because the joining strength between the directly joined substrates is high, and the joined interface is a mirror surface.
The reflected bulk waves do not cause a problem where the resonators arranged to overlap each other in the SAW propagating direction are at a sufficient distance from each other. In a small-sized SAW device chip that is often seen today, however, the distance between each two neighboring resonators in the SAW propagating direction is too short to prevent the reflected bulk waves from entering the neighboring resonators in the SAW propagating direction. As a result, spurious waves are generated, and the filter characteristics deteriorate.